1. Field of the Invention
The invention is related to the field of blow molding, and in particular, to a ring shaped valve piston around the stretch rod/blow nozzle hole of a blow molding machine.
2. Background of the Invention
Blow molding uses low and high pressure air to expand a pre-form part in a mold cavity. Typically the blow molding process uses 4 valves, a high pressure valve, a low pressure valve and two exhaust valves. The high and low pressure valves are known as the blow valves because these valves supply the air used to “blow” the pre-form into its final shape. The valves are typically connected to a valve block and the valve block feeds the blow nozzle that forms a seal against the pre-form/bottle inlet. The space between the outlets on the blow valves and the pre-form/bottle inlet is known as dead space. The dead space is filled, first with low pressure air, and then with high pressure air, for each blow mold cycle. The low and high pressure air must also be vented from the dead space during each blow molding cycle.
Older blow molding machines connected the blow valves to the valve block using low and high pressure lines. The pressure lines and the passageways in the valve block created large dead spaces, causing slow cycle times. Some newer blow molding machines attached the blow valves and exhaust valves directly to the valve block, eliminating the pressure lines between the valves and the valve block and thereby reducing the dead space. An example of a blow molding machine that attaches the blow valves and exhaust valves directly to the valve block is disclosed in U.S. Pat. No. 6,905,326 to Voth et al. entitled “Blow molding machine comprising control valves, which are mounted on the blowing device and which control the blowing air” which is hereby incorporated by reference. These newer blow molding machines still have dead space formed along the entire length of the passageways formed in the valve block (also called a valve carrier) that connect the blow valves to the blow nozzle.